We report evolving pore formation in both surface and interior structures of polystyrene (PS) fibers electrospun from solutions of dimethylformamide (DMF), as low, and tetrahydrofuran (THF), as high volatile solvents. The environmental conditions, i.e., temperature and relative humidity of electrospinning process were systematically changed using a high accuracy conditioning box to provide a comprehensive investigation of morphology evolution within electrospun fibers. Three-angle phase diagrams of H2O/DMF/PS and H2O/THF/PS systems were constructed at temperature of 20, 40, and 60 degrees C by measuring the cloud points as well as calculating the free energy of mixing, in order to analyze the thermodynamic instability of polymer solution jet undergoing solvent evaporation and water penetration. According to the results, pores form within the fiber if DMF has low volatility and develop onto the fiber surface if THF high volatile solvent is employed. In the former case vapor-induced phase separation is involved while in latter case mechanism of breath figure formation is dominant. Three types of morphology for fiber surface, i.e., non-porous, porous and wrinkled are recognized as relative humidity and temperature of electrospinning process vary. Morphology transition of surface fiber is taken place at certain relative humidity while it shifts by varying the temperature. Superimposing the relative humidity and temperature not only describes the suppression of bead formation but also controls the mean diameter of electrospun fiber. (C) 2012 Elsevier Ltd. All rights reserved.